Funds are requested for a Nordion Gammacell 40 research irradiator. The irradiator contains dual 1,800 Ci Cs-137 sources and produces a central dose rate of 1.3 Gy/min. [unreadable] 15%. Typical dose uniformity is [unreadable] 6.5% over a 260 mm diameter and a 100mm height. The Gamma cell 40 has a chamber size of 312 mm by 100 mm. The Nordion attenuator set will also be purchased which will allow us to reduce the dose rate of irradiation by 80, 60, or 33% so that dose rates can be varied for different applications. The irradiator will be used to both irradiate cells, such as spleen cells to be used as a source of antigen-presenting cells, and to ablate bone marrow cells of mice to make them receptive to donor bone marrow engraftment (chimeras). Currently there is no irradiator available at Montana State University and the closest irradiator we know of is at the NIAID Rocky Mountain Laboratories in Hamilton, Montana, which is about 260 miles from our campus. Thus, this irradiator is needed for us to take advantage of state-of-the art immunologic techniques as well as to take full advantage of the potential use of transgenic mice in biomedical investigations. [unreadable] [unreadable]